A prior patent to A. P. McCandlish et al., U.S. Pat. No. 2,874,641, discloses a pump jack which is believed to be the closest approach to the invention herein disclosed. In this patent, a head is attached to the piston rod of an upright hydraulic cylinder, the head providing a bearing for a cross shaft to which a pair of sprockets are attached. A chain is trained over each sprocket, one reach of the chain extending down to a crosshead to which the polish rod is attached, and the other reach of each chain extending down to a stationary support on the tower.
When the rod of the cylinder in the McCandlish patent is moved upwardly to elevate the polish rod and the string of sucker rods attached thereto, a very great amount of stress is applied to the sprockets since they must not only rotate but also move bodily upwardly. Further, since the piston rod of the McCandlish pump jack is extended when it lifts the polish rod, great stress is applied to the rod bearings.
In contrast, in my improved pump jack, the upper sprockets do not move bodily, but merely rotate. Further, to elevate the polish rod, the piston rod is drawn into the hydraulic cylinder and the rod bearings are therefor not subjected to the great amount of stress, as before mentioned.
The pump jack of my invention is of simplified construction whereby it may be manufactured and sold at a comparatively low cost. The pump jack is mounted directly upon the head of an oil well casing so that it is unnecessary to provide special location preparation. The improved pump jack is fully automatic in operation and includes simplified controls to vary the acceleration or deceleration of the pumping stroke and to eliminate the "hammering" effect of high pressure hydraulic systems upon rapid reversal of the flow of the operating fluid.